Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
The first obvious difference between Mint and Ubuntu is of course Mint's artwork, which , I must say, is a lot more appealing than Ubuntu's brown and orange. Ubuntu's Human theme is a regular discussion starter, and personally, I do not like it at all. For some reason, brown and orange just do not work for me as the contrastive colours in a user interface. Sure, it helped define the brand "Ubuntu", but the first thing I do after an Ubuntu installation is applying my own combination of window decor, icon set, and interface theme. Mint uses the more traditional (dark) blue as its contrastive colour, with elements of mint green (really?) scattered throughout the desktop (icons, mostly).
They also applied some other cool tricks like diagonal striping on menu bars, menu item highlights, and the GNOME panel, which looks pretty slick. It is also refreshing to see UI people who are actually not trying to 'unify' everything. Unifying the title, menu, and toolbars seems to be all the rage these days, but I detest it as it seriously hinders differentiation between the various elements. On Mint's default theme, the title, menu, and toolbars are clearly separate entities.
Mint also comes with a few configuration tools you will not see in Ubuntu. MintInstall allows you to download .mint files from the Mint Software Portal, which act as metapackages for all sorts of applications, such as Real Player, Opera, Evolution, and more. MintDesktop is a configuration tool where you can configure some GNOME options that would normally require gconf-editor (Nautilus mode, desktop items). It also turns on and off the Network Autobrowsing feature, a great tool that automatically mounts SAMBA machines on your network.